1. Field of the Invention
This invention relates to an instrument probe assembly and, in particular, to an instrument probe assembly ideally suited for a pH cell used in liquids containing debris and suspended foreign matter.
2. Description of the Prior Art
Considerable difficulty is experienced in the measurement of variables of a liquid stream which contains suspended solids and debris. This problem is particularly acute with instruments having sensing elements such as electrodes which generate a signal proportional to the exposed surface of the element, as typical of electrodes of a pH meter having a reference electrode with a porous junction and a sensing electrode that are in contact with the process liquid. Fouling of the electrode or junction surfaces with debris and foreign matter from the liquid interferes with the response of the electrodes and causes erroneous determinations.
Some attempts have been made to obviate the difficulties by providing a perforated circular shield around the sensing electrode and reference junction of a probe assembly for a pH meter. When this device is employed with a liquid stream having a substantial quantity of suspended debris such as a raw sewage stream, the cylindrical shield rapidly fills with solids, clogging the electrode surfaces and requiring frequent maintenance and cleaning.